Process for making ti-ni based functionally graded alloys and ti-ni based functionally graded alloys produced thereby

ABSTRACT

The present invention relates to Ti—Ni based functionally graded alloys easy in proportional control, which are made by cold working and annealing Ti—Ni based alloys under a predetermined temperature gradient. The thus processed Ti—Ni based functionally graded alloys have a shape memory effect and an ultra elasticity and at the same time, have a consecutive variation of shape depending on a temperature variation.

TECHNICAL FIELD

The present invention relates to Ti—Ni based functionally graded alloys, and more particularly, to a process for making Ti—Ni based functionally graded alloys and Ti—Ni based functionally graded alloys produced thereby, in which Ti—Ni based alloys are cold worked, annealed under a predetermined temperature gradient, and functionally graded.

BACKGROUND ART

Ti—Ni based shape memory alloys are rapidly deformed at a Martensite transformation start temperature (Ms) if being cold worked, annealed at a predetermined temperature, and cooled with being given a load. After that, if a temperature elevates, a recovery of rapid deformation occurs at an austenite deformation start temperature (As) (Referring to FIG. 1). As such, shape memory alloys are being applied to various on-off switching actuators using a phenomenon in which a rapid deformation is generated at a specific temperature and recovered.

In case where the Ti—Ni based shape memory alloys are applied as an actuator element for robot, they have to enable a position control by proportional control. However, because the conventional Ti—Ni based shape memory alloys are rapidly deformed at a specific temperature, they were appropriate for an on-off switching actuator, but were inappropriate for a proportional control actuator.

The present invention is a result of making efforts to fix the defects in which the shape memory alloys are rapidly deformed at a specific temperature.

DISCLOSURE OF INVENTION Technical Problem

Accordingly, the present invention is directed to a process for making Ti—Ni based functionally graded alloys and Ti—Ni based functionally graded alloys produced thereby that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a process for making Ti—Ni based functionally graded alloys, for facilitating a proportional control by consecutively varying a transformation temperature (Ms and As) within the same Ti—Ni based alloys and generating gradual deformation over a wide range of temperature (Referring to FIG. 2).

Technical Solution

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, there is provided a process for making Ti—Ni based functionally graded alloys, wherein Ti—Ni based alloys are cold worked, annealed under a temperature gradient, and given functionally grading.

Cold working may be performed by 25% to 65% and annealing may be performed under a temperature gradient of 823 K to 466 K.

In another aspect, there is provided Ti—Ni based functionally graded alloys enabling a proportional control, made according to the process.

A deformation-rate recovery speed of the alloys may be reduced to 1/30 to 1/100.

Advantageous Effects

Ti—Ni based alloys processed according to the present invention has a shape memory effect and an ultra elasticity and at the same time, a deformation-rate recovery speed gets smaller 1/30 to 1/100 compared to that of conventional alloys. The Ti—Ni based alloys having a low deformation-rate recovery speed is easy in position control through proportional control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating a deformation-temperature curve for shape memory alloys heat-treated at a predetermined time;

FIG. 2 is a graph illustrating a deformation-temperature curve for functionally graded shape memory alloys;

FIG. 3 shows differential scanning thermal analysis curves for Ti-50.0Ni (at %) alloys processed by solution heat treatment and Ti-50.0 (at %) alloys processed by a temperature gradient heat treatment;

FIG. 4 is a result of arrangement of Ms-by-positions of a wire obtained by 25% cold working and annealing Ti-50.0Ni (at %) alloys under a temperature gradient of 658 K to 466 K according to the present invention;

FIG. 5 is a result of arrangement of Ms-by-positions of a wire obtained by 25% cold working and annealing Ti-50.0Ni (at %) alloys under a temperature gradient of 823 K to 658 K according to the present invention;

FIG. 6 is a result of arrangement of Ms-by-positions of a wire obtained by 65% cold working and annealing Ti-50.0Ni (at %) alloys under a temperature gradient of 823 K to 658 K according to the present invention;

FIG. 7 is a graph illustrating a deformation (e)-temperature (T) curve of Ti-50.0Ni (at %) alloys processed by solution heat treatment according to the present invention;

FIG. 8 is a graph illustrating a deformation (e)-temperature (T) curve of a wire obtained by 25% cold working and annealing Ti-50.2 (at %) alloys under a temperature gradient of 658 K to 466 K according to the present invention; and

FIG. 9 is a graph illustrating a deformation (e)-temperature (T) curve of a wire obtained by 65% cold working and annealing Ti-50.2 (at %) alloys under a temperature gradient of 658 K to 466 K according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to accompanying drawings.

First Exemplary Embodiment

FIG. 3( a) shows a differential scanning thermal analysis curve of Ti-50.0Ni (at %) alloys processed by solution heat treatment. Here, it can be appreciated that a peak appears one by one during cooling and heating. This is caused by B2 (Cubic)-B19′ (Monoclinic) Martensite transformation.

FIGS. 3( b), 3(c), 3(d), and 3(e) show results of differential scanning thermal analysis of samples taken at positions shown in FIG. 3( f) from Ti-50.0Ni alloys that are 25% cold worked and annealed under a temperature gradient of 658 K to 466 K. Wide peaks were observed at the time of cooling and heating. Particularly, it can be appreciated that a peak of a sample taken in a low annealing temperature region is wider.

Ti-50.0Ni (at %) alloys were cold worked within a range of 25% to 65% before being heat-treated under a temperature gradient. This is because if cold working is performed at less than 25%, a characteristic of functionally grading cannot be obtained due to a small variation of temperature after heat treatment and cold working of more than 65% is impossible. The temperature gradient heat treatment was performed using a heat treatment furnace having a temperature gradient of 823 K to 466 K. However, in the case of 65% cold working, a heat treatment was performed at a temperature gradient of 823 K to 658 K. This is because if a heat treatment is performed at a temperature gradient of 658 K to 466 K, a deformation ratio is small less than 1% and thus is inappropriate for an actuator element.

In an exemplary embodiment of the present invention, Ti-50.0Ni (at %) alloys were used as Ti—Ni based alloys. However, in addition, other Ti—Ni based alloys can be also used and even in such a case, a similar result can be obtained.

Mode for the Invention Second Exemplary Embodiment

Ti-50.0Ni (at %) alloy wire having a length of 150 mm is 25% cold worked and annealed under a temperature gradient of 658 K to 466 K. A sample was taken at an interval of 5 mm and processed by differential scanning thermal analysis. A result of arrangement of measured Ms was shown in FIG. 4. It can be appreciated that Ms consecutively varies as a position varies. In case where a wire having a length of 150 mm is 25% cold worked and annealed under the temperature gradient of 658 K to 466 K, a variation of Ms is equal to about 19 K.

Third Exemplary Embodiment

Ti-50.0Ni (at %) alloy wire having a length of 150 mm is 25% cold worked and annealed under a temperature gradient of 823 K to 658 K. A sample was taken at an interval of 5 mm and processed by differential scanning thermal analysis. A result of arrangement of measured Ms was shown in FIG. 5. It can be appreciated that Ms consecutively varies as a position varies. In case where a wire having a length of 150 mm is 25% cold worked and annealed under the temperature gradient of 823 K to 658 K, a variation of Ms is equal to about 14 K.

Fourth Exemplary Embodiment

Ti-50.0Ni (at %) alloy wire having a length of 150 mm is 65% cold worked and annealed under a temperature gradient of 823 K to 658 K. A sample was taken at an interval of 5 mm and processed by differential scanning thermal analysis. A result of arrangement of measured Ms was shown in FIG. 6. It can be appreciated that Ms consecutively varies as a position varies. In case where a wire having a length of 150 mm is 65% cold worked and annealed under the temperature gradient of 823 K to 658 K, a variation of Ms is equal to about 60 K.

As described above, it could be confirmed from the exemplary embodiments 2 to 4 and FIGS. 4 to 6 that cold working and annealing under a temperature gradient can cause a consecutive variation of a transformation temperature within the same wire and plate.

Fifth Exemplary Embodiment

A deformation (e)-temperature (T) curve for Ti-50.0Ni (at %) alloys processed by solution heat treatment was shown in FIG. 7. If alloys are cooled under a load strain of 80 MPa, there occurs deformation at a temperature expressed by Ms. This is caused by B2 (Cubic)-B19′ (Monoclinic) Martensite deformation. Meantime, if alloys are heated, deformation is recovered at a temperature expressed by As. This is caused by B19′-B2 austenite deformation. A deformation-rate recovery speed (dε/dT) at the time of heating is equal to about 1%/K.

Sixth Exemplary Embodiment

Ti-50.0 (at %) alloy wire was 25% cold worked and annealed under a temperature gradient of 658 K to 466 K. A deformation (ε)-temperature (T) curve of the processed wire was shown in FIG. 8. If alloys are cooled under a load strain of 80 MPa, there occurs deformation at a temperature expressed by Ms. This is caused by B2 (Cubic)-B19′ (Monoclinic) Martensite deformation. Meantime, if alloys are heated, deformation is recovered at a temperature expressed by As. This is caused by B19′-B2 austenite deformation. A deformation-rate recovery speed (dε/dT) at the time of heating is equal to about 0.03%/K.

Seventh Exemplary Embodiment

Ti-50.0 (at %) alloy wire was 65% cold worked and annealed under a temperature gradient of 658 K to 466 K. A deformation (ε)-temperature (T) curve of the processed wire was shown in FIG. 9. If alloys are cooled under a load strain of 80 MPa, there occurs deformation at a temperature expressed by Ms. This is caused by B2 (Cubic)-B19′ (Monoclinic) Martensite deformation. Meantime, if alloys are heated, deformation is recovered at a temperature expressed by As. This is caused by B19′-B2 austenite deformation. A deformation-rate recovery speed (dε/dT) at the time of heating is equal to about 0.01%/K. If the 65% cold worked alloys had been heated within a range of temperature of 658 K to 466 K, they were inappropriate for an actuator element because a deformation rate is so small less than 1%.

As described above, it can be appreciated from the exemplary embodiments 5 to 7 and FIGS. 7 to 9 that if Ti—Ni based alloys are cold worked and annealed under a temperature gradient, a deformation-rate recovery speed is equal to 0.03%/K to 0.01%/K and gets smaller about 1/30 to 1/100 compared to a recovery speed of 1%/K when being annealed at a predetermined temperature. Accordingly, it can be appreciated that Ti—Ni based alloys for proportional control can be made in a method of annealing under a temperature gradient after cold-working.

INDUSTRIAL APPLICABILITY

Ti—Ni based alloys processed according to the present invention has a shape memory effect and an ultra elasticity and at the same time, a deformation-rate recovery speed gets smaller 1/30 to 1/100 compared to that of conventional alloys. The Ti—Ni based alloys having a low deformation-rate recovery speed is easy in position control through proportional control and therefore, is useful for an industrial field requiring a precise position control such as an actuator for robot.

While the present invention has been described and illustrated herein with reference to the preferred embodiments thereof, it will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention that come within the scope of the appended claims and their equivalents. 

1. A process for making Ti—Ni based functionally graded alloys, wherein Ti—Ni based alloys are cold worked, annealed under a temperature gradient, and given functionally grading.
 2. The process of claim 1, wherein cold working is performed by 25% to 65% and annealing is performed under a temperature gradient of 823 K to 466 K.
 3. Ti—Ni based functionally graded alloys enabling a proportional control, made according to the process of claim
 1. 4. The alloys of claim 3, wherein a deformation-rate recovery speed of the alloys is reduced to 1/30 to 1/100.
 5. Ti—Ni based functionally graded alloys enabling a proportional control, made according to the process of claim
 2. 